An optical encoder of the related art is known. The optical encoder includes a scale provided with a diffraction grating arranged along a measurement direction and a head including a light-receiving unit that receives light emitted from a light source through the diffraction grating. The head is configured to move relative to the scale along the measurement direction thereof and detect an amount of relative movement with the scale.
In this optical encoder, the diffraction grating transforms the light emitted from the light source into a plurality of beams of diffracted light. The plurality of beams of diffracted light produce an interference pattern having the same period as that of the diffraction grating, and the light-receiving unit detects a signal from this interference pattern. The optical encoder calculates an amount of relative movement between the scale and the head from the detection result (signal) from the light-receiving unit.
The plurality of beams of diffracted light include diffracted light traveling in the same direction as the optical axis of the light emitted from the light source, diffracted light traveling at predetermined diffraction angles on both sides of the optical axis, and diffracted light traveling at diffraction angles greater than the predetermined diffraction angles on both sides of the optical axis.
Assuming the diffracted light traveling in the same direction as the optical axis is zeroth-order diffracted light, the plurality of beams of diffracted light can be ordered as ±first-order diffracted light and ±second-order diffracted light, moving from the zeroth-order diffracted light in the direction in which the diffraction angle increases.
The light-receiving unit primarily detects a signal from an interference pattern produced from the ±first-order diffracted light. Accordingly, the ±first-order diffracted light becomes signal diffracted light, and diffracted light of higher orders than the ±first-order diffracted light becomes noise diffracted light.
When the light-receiving unit is irradiated with signal diffracted light and noise diffracted light, the interference pattern produced by the signal diffracted light is distorted by the noise diffracted light and noise arises in the signal detected by the light-receiving unit. There is thus a problem in that the accuracy of the amount of relative movement calculated from the signal by the light-receiving unit drops and the optical encoder is less reliable.
In response to this, an interference-type position measurement device (optical encoder) disclosed in Patent Document 1 includes a main scale (a scale) arranged along a transmission-type diffraction grating (a diffraction grating), a light source that irradiates the main scale with light, and a photodetector (a light-receiving unit) that outputs a signal from an interference pattern produced by a plurality of diffracted light beams that have passed through the transmission-type diffraction grating.
The interference-type position measurement device is configured such that the main scale is arranged between the light source and the photodetector. Additionally, the interference-type position measurement device includes, between the light source and the main scale, a diffraction grating beam splitter and an optical block that irradiates the main scale with only the ±first-order diffracted light of the plurality of diffracted light beams passing through the diffraction grating beam splitter from the light source.
The optical block includes an integrated prism that reflects the ±first-order diffracted light toward the main scale but refracts the diffracted light aside from the ±first-order diffracted light in a direction where the main scale is not irradiated, and a zeroth-order diffracted light shielding device that physically shields zeroth-order diffracted light (that is, a diffracted light shielding device that shields diffracted light aside from ±first-order diffracted light). The integrated prism is formed as a rectangular parallelepiped that takes a direction parallel to the optical axis of the light emitted toward the diffraction grating beam splitter as a longitudinal direction. The zeroth-order diffracted light shielding device is provided within the integrated prism, and is arranged in substantially the central part of the integrated prism.
The interference-type position measurement device includes the optical block, and thus removes noise diffracted light using the zeroth-order diffracted light shielding device and the integrated prism so as to irradiate the main scale with only the ±first-order diffracted light (signal diffracted light).